The invention relates to a method for generating extreme ultraviolet/soft x-ray radiation by means of gas discharge, in particular, for EUV lithography, wherein in a discharge vessel two electrodes are connected to high voltage, between which in an area of two electrode recesses that are coaxial to one another a gas fill with predetermined gas pressure in accordance with a discharge operation realized on the left branch of the Paschen curve is provided, in which a plasma emitting the radiation is formed when supplying energy.
Preferred fields of application for the extreme ultraviolet (EUV) radiation or the soft x-ray radiation in the range of approximately 1 nm to 20 nm wavelength are particularly the EUV lithography. WO 99/29145 discloses a method having the features of the aforementioned kind. The device used in this method is comprised of an anode with a central bore recess and an oppositely positioned hollow cathode. The device operates in an environment of constant gas pressure. For generating EUV radiation, gases of the elements of the atomic number Z>3 are preferred, for example, Xe with broadband emission characteristics. When high voltage is employed, gas firing occurs depending on the pressure and the electrode distance. The pressure of the gas and the electrode distance are selected such that the system operates on the left branch of the Paschen curve and, as a result of this, no dielectric firing between the electrodes occurs. Only in the vicinity of the hollow cathode the field lines are sufficiently extended so that the firing conditions are fulfilled above a certain voltage. A current-conducting plasma channel of axially symmetrical shape is then formed between the electrodes in accordance with the electrode recess. The electrical circuit connected to the device is configured such that a very high discharge current occurs when the current-conducting channel is generated. This current generates a magnetic field about the current path. The resulting Lorentz force constricts the plasma. It has been known for a long time that this constriction effect can cause the plasma to be heated to very high temperatures and can generate radiation of very short wavelength. It has been demonstrated that the device can generate EUV light (10 to 20 nm) very effectively, that it enables high repetition frequencies, and has a moderate electrode wear.
The plasma emitting in the short-wave range is produced along an axis of symmetry in the area of the hollow cathode and past the recess of the anode, depending on the provided conditions. Relevant parameters for a geometry of the plasma are derived from the shape of the electrodes, as well as parameters of a supplied electrical current, its duration, its shape, and its amplitude, as well as the gas pressure conditions and the composition of the gas of the gas fill in the discharge vessel or in the area of the electrodes.
The known method results in a pinch, i.e., in a plasma channel that should be shorter and whose radiation should be able to be decoupled from the electrode system in a better way.